It is known that an air cylinder which should not be lubricated is subject to some wear on that account. However, lubricant which is sometimes unavoidably entrained in the air gathers in the asperities of the cylinder wall and will provide adequate lubrication nonetheless, provided that such asperities are adequate in number, size and depth for that purpose. That is, if the cylinder wall is ordinarily smooth, whatever lubricant is deposited is promptly instead wiped to either end of the cylinder by the piston seal ring.
It is an object of the present invention to provide the equivalent of such asperities in a glass filament reinforced epoxy resin body of an air cylinder.
U.S. Pat. No. 3,616,000 of Butzow and Harris discloses a method of making a wound glass filament reinforced epoxy resin tube with a fabric liner of low friction material exposed at the inner surface of the tube. The tube is then cut into lengthwise sections which may comprise any number of sleeve bearings. U.S. Pat. No. 3,804,479 of Butzow and Harris discloses a fabric which is particularly suitable for use as the fabric liner referred to and includes warp yarns of the polytetrafluoroethylene resin marketed under the trademark "Teflon." Bearings made in accordance with U.S. Pat. No. 3,616,000 have met with great success with various low-friction fabrics. However, a peculiarity of such bearings when made with the fabric of U.S. Pat. No. 3,804,479 and under certain conditions has been the presence of a barely visible series of closely spaced circumferential grooves in the bearing face. The certain conditions referred to obtain when the viscosity of the resin is lower than average and the filaments of the warp are more loose than might be desirable for the manufacture of such sleeve bearings.
The grooves are actually a single, spiral groove and are of such little depth that they disappear after only a moderate amount of break-in wear. Essentially they are filled by the intermediate material which is worn away and is then overlaid by the Teflon which makes such bearings self-lubricating and capable of being operated without an applied lubricant. Inspection at 40.times. magnification suggests that the spiral groove is formed by the capillary interaction of the resin with the yarn of the fabric liner and the non-wetting parting agent which is applied to the mandrel. It is believed that small voids develop due to such action where the yarns of the fabric curve away from the mandrel and that where these voids are sufficiently close together they tend to merge. Their locations, of course, follow the weave of the fabric and thus the voids tend to merge in lines as will be described.
It has been found that an air cylinder tube of a construction similar to that of such bearings having such a spiral groove will advantagously retain the lubricant deposited on the cylinder wall and that such a groove or grooves usefully remain intact for a significant period of service. That is, they are not promptly filled as they are in the bearings because the amount of movement of the piston over the grooves is comparably far less than that of the bearing shaft, the side loading of the piston is generally nominal or at least of a comparatively low amount, and the lubricant itself reaches the wear of the surfaces between the grooves. In fact, the entrained lubricant referred to is generally such that the lubricious effect of the Teflon filaments is only required where the side-loading of the piston against the cylinder wall may be such as 50 psi. (325 cm.sup.2). Thus, the fabric lining in most instances does not require the use of the relatively expensive Teflon material.
While a spiral groove is generally formed by the merger of the voids, such a groove is not necessarily a requirement in carrying out the present invention. That is, a series of voids which have not merged to form a continuous groove should in some cases be adequate to retain the lubricant as required.